244 



PROCEEDINGS OF THE ACADEMY OF 



plants are adaptations to external needs or forces, — and of the former a large 

 proportion are of the same kind. How then could they owe their existence to 

 a process regulated by adaptation ? 



Darwin is aware of these facts to some degree, but, as already said, he does 

 not dwell on them. Where he does, he does not attjpipt to account for them 

 on the principle of natural selection. 



There are, it appears to us, two laws of means and modes of development, 

 I. The law of acceleration and retardation. II. The law of natural selection. 



It is my purpose to show that these propositions are distinct, and not one 

 apart of the other: in brief, that while natural selection operates by the 

 " preservation of the fittest," retardation and acceleration act without any 

 reference to " fitness " at all ; that instead of being controlled by fitness, 

 it is the controller of fitness. Perhaps all the characteristics supposed to 

 mark generalized groups from genera up (excepting, perhaps, families), to 

 have been evolved under the first mode, combined with some intervention of 

 the second, and that specific characters or species have been evolved by a com- 

 bination of a lesser degree of the first with a greater degree of the second mode. 



1 propose to bring forward some facts and propositions in the present essay 

 illustrative of the first mode. 



I. On. the relations of nearly allied genera. 



First. The writer's views of the relations of genera have already been given 

 at the close of an Essay on the Cypriuoid Fishes of Pennsylvania.* It is easy 

 enough to define isolated genera which have few immediate affines, but among 

 extensive series of related forms the case is different. One principle, however, 

 pervades the conception and practice of all zoologists and botanists, which 

 few take pains to analyse or explain. It is simply that they observe a succes- 

 sional relation of groups, by which they pass from one type of structure to one 

 or several other types, and the presence or absence of the steps in this succes- 

 sion they regard as definitions of the genera. 



It is true that the reader will often find introduced into diagnoses of 

 genera, characters which indicate nothing of this sort. It is often necessary, 

 indeed, to introduce characters which ari not peculiar to the genus character- 

 ized, for the sake of distinguishing it from similar ones of other series, but this 

 only in an imperfect state of the record. Moreover, the ability of the writer 

 to distinguish genera being thus tested, he too often fails by introducing 

 family and specific characters, or by indulging in an unnecessary redundancy. 

 In general it may be said that adjacent genera of the same series differ from 

 each other by but a single character; and generally, that the more remote 

 differ by characters as numerous as the stages of their remove. 



It is precisely as, among the inorganic elements, we pass from the electro- 

 negative, non-oxidizing extreme of the Halogens, with Fluorine as the extreme, 

 to the electro-positive, violently oxidizing extreme of the alkaline metals, 

 whose extreme is potassium, by steps whose relative position is measured or 

 determined first by these tests ; and as these steps have each their included 

 series of bodies, characterized by their successive relations on the lower level 

 of a subordinate range of characters. This principle is distinctly admitted by 

 many zoologists ;f those who deny it generally failing to perceive it because 

 they attempt to guage a major scale by characters which are really the test of 

 one or all of the subordinate or included scales. It holds true of most of the 

 groups of organic beings ; thus the class is a scale of orders, the order of 

 tribes. I will not now say that the tribe is a scale of families, as the case is 

 here much modified, but what is chiefly to be considered in this essay, is that 

 the family is composed of one or several scales of genera. 



♦Trans. Amer. Philos. Soe., 1866, from Proc. Aead. Nat. Sci., Phil., 1859, 332. 

 fProf. Bronn, in his Classen u. Ordnungen des Thierreiches, has everywhere a chapter 

 on Die aufsteigende Reihe, — "the ascending scale." 



[Oct. 



